Optical fibers are quickly replacing copper cable as the transmission medium for communication systems, such as the long-distance and local telephone networks and as interconnects within a computer system. The extremely wide bandwidth of optical fibers allows the optical carrier to be modulated at very high data rates. Available electronic hardware allows data signals to be modulated on one end of the fiber and detected on the other end at data rates of nearly 10 Gbit/sec.
The absorption of optical fibers has been reduced to the point where an optical signal can propagate for hundreds of kilometers on an optical fiber without the need for intermediate amplification or regeneration. However, the combination of long distance and high data rates presents the problem of dispersion on the fiber. An optical signal, usually in the form of well defined pulses at the transmitting end, may contain a number of spatial modes. Different spatial modes represent the allowed eigenmodes for propagation of electromagnetic radiation on the fiber, e.g., the TE.sub.mn and TM.sub.mn modes. Each of these modes may have a slightly different propagation velocity on the fiber. Therefore, the optical signal that was transmitted as a sharp pulse has its modes spatially dispersed on a long fiber. The spatial dispersion translates to a temporal dispersion at the receiver so that the pulse is received in a distorted form. The dispersion limits the data rates of signals to be accurately transmitted on optical fibers. Multi-mode fibers, that is, those that transmit higher-order modes, permit only very limited distances for transmission or very reduced data rates.
The dispersion problem itself has been brought well under control. Most fibers being installed in the field are single-mode fibers that have a cut-off frequency between the two lowest-order modes. That is, the lowest-order mode propagates along the fiber, while all of the higher-order modes are quickly attenuated. For a step-index fiber having a core diameter .alpha., numerical aperture NA, and carrying radiation at a wavelength .lambda., there is defined a normalized frequency parameter EQU V=2.pi.NAa/.lambda..
The number of guided spatial modes is EQU N=V.sup.2 /2.
A step-index fiber become single-mode when EQU V&lt;2.405.
With only a single spatial mode propagating on the fiber, there is no spatial mode dispersion. This and other aspects of optical fibers are discussed by S. D. Personick in his book Fiber Optics: Technology and Applications, Plenum Press, 1985, pages 6-45.
However, in the cylindrically symmetric geometry of an optical fiber, the lowest order mode is two-fold degenerate, that is, the TE.sub.00 mode is both orthogonal and equivalent to the TM.sub.00 mode. These two degenerate modes are usually represented by the electric polarization of the propagating wave. For example, a light wave propagating on an optical fiber lying in the z-direction may have its electric vector lying in the x-direction or in the y-direction or some combination of the two. In the present state of optical fiber technology, there is no control in long fibers over the distribution of the optical power between the two polarization modes. As a fiber goes around a bend, the fiber becomes birefringent, and a previously well defined single polarization mode is transformed into a combination of the two polarization modes. Indeed, the transformations between the two modes appear to depend upon uncontrolled environmental factors which change over time. Therefore, the light wave arriving at the receiver is of unknown, uncontrolled, and temporally varying polarization.
The lack of polarization control would present no problem if the receiver were independent of polarization, that is, polarization insensitive. For example, most optical detectors, such as a PIN detector, are polarization insensitive. However, many modern fiber optic systems transmit multiple signals at different optical frequencies on one fiber. A wavelength-division multiplexing (WDM) system uses multiple laser transmitters lasing at different wavelengths and modulated by different data signals. Their outputs are combined on a single fiber for long-distance distribution. In one WDM architecture, the combined WDM signal is distributed to multiple receivers so that each receiver simultaneously receives a large number of data signals. The receiver then optically filters the entire WDM signal to pass only the desired channel to a wideband photodetector.
The optical filter for such an application should have a narrow passband (.about.1 nm), be electronically tunable over a wide bandwidth, and be economical. Two candidates are acousto-optic filters and liquid-crystal Fabry-Perot etalon filters. Unfortunately, both types of filters are usually designed to operate upon one polarization state of light and will accurately filter only that polarization. Their polarization sensitivity causes operational problems when used in receivers for optical fibers in which there is no control of polarization. With effort, these types of filters may be made insensitive to polarization. Heffner et al. have disclosed a multi-stage polarization-insensitive acousto-optic filter in U.S. Pat. No. 5,002,349. Patel has disclosed a polarization-insensitive liquid-crystal filter in U.S. patent application, Ser. No. 5,068,749, filed Aug. 31, 1990. However, their structures are complex and thus expensive.
A different, systems-level approach to the polarization problem has been disclosed by Hodgkinson et al. in "Polarisation insensitive heterodyne detection using polarization scrambling," Postdeadline Papers: OFC/IOOC '87, Reno, Nev., Jan. 1987, paper PDP15-01, pages 62-65. Their method intentionally provided equal amounts of orthogonal polarizations on the transmitting side. After the optical signal had been data modulated at 20M bit/sec, a LiNbO.sub.3 phase modulator polarization scrambled it at 80 MHz so that equal power was given to the orthogonal polarization modes within each data bit. That is, their transmitter polarization scrambled at a rate faster than the data rate. Although such an approach circumvents the slow variations in the fiber's polarization characteristics, the transmitter becomes complex.
Tokuda et al. have considered the problem of launching a controlled distribution of modes onto a multi-mode fiber to be evaluated in "Measurement of baseband frequency response of multimode fibre by using a new type of mode scrambler," Electronics Letters, volume 13, 1977, pages 146-147. They guaranteed a uniform mode distribution by wrapping the input end of the multi-mode fiber on a multi-mode scrambler. Their scrambler consisted of the fiber wrapped on a row of closely spaced columns such that the fiber was bent in a corrugated shape. A commercial version of this multi-mode scrambler is available from Newport as Model FM-1. Other mode scramblers are disclosed by D. Marcuse in the book Principles of Optical Fiber Measurements, Academic Press, 1981, pages 201-203, for example, a graded-index fiber spliced on either end to a step-index fiber. Even a long length of multi-mode fiber can scramble modes. As far as is known, the problem of polarization scrambling in a passive device has not been addressed. The effectiveness of prior-art multi-mode scramblers for polarization scrambling has not been completely understood.